I have a friend that had a broken original BMW radio (525tds), the volume always went to the maximum on the left speaker.
The problem is that a storage IC in the radio was broken, few bits in the I2C always returned 1, so that causes the radio to go max volume and on the left speaker.
The IC is located inside the radio on the main baord. The solution was solder out the old IC (24C16), read it out with an IC programmer, take a new
24C16 and program this with the old values. Solder it back again. Everything worked ok again.
Hope this helps when one of you has similar problems!